The epithelial cells lining the lumenal side of the gastrointestinal tract (GIT) can be a major barrier to drug delivery via oral administration. However, there are four recognized transport pathways which can be exploited to facilitate drug delivery and transport: the transcellular, paracellular, carrier-mediated and transcytotic transport pathways. The ability of a drug, such as a conventional drug, a peptide, a protein, a macromolecule or a nano- or microparticulate system, to “interact” with one or more of these transport pathways may result in increased delivery of that drug from the GIT to the underlying circulation.
Certain drugs utilize transport systems for nutrients which are located in the apical cell membranes (carrier mediated route). Macromolecules may also be transported across the cells in endocytosed vesicles (transcytosis route). However, many drugs are transported across the intestinal epithelium by passive diffusion either through cells (transcellular route) or between cells (paracellular). Most orally administered drugs are absorbed by passive transport. Drugs which are lipophilic permeate the epithelium by the transcellular route whereas drugs that are hydrophilic are restricted to the paracellular route.
Paracellular pathways occupy less than 0.1% of the total surface area of the intestinal epithelium. Further, tight junctions, which form a continuous belt around the apical part of the cells, restrict permeation between the cells by creating a seal between adjacent cells. Thus, oral absorption of hydrophilic drugs such as peptides can be severely restricted. Other barriers to absorption of drugs may include hydrolyzing enzymes in the lumen brush border or in the intestinal epithelial cells, the existence of the aqueous boundary layer on the surface of the epithelial membrane which may provide an additional diffusion barrier, the mucus layer associated with the aqueous boundary layer and the acid microclimate which creates a proton gradient across the apical membrane. Absorption, and ultimately bioavailability, of a drug may also be reduced by other processes such as P-glycoprotein regulated transport of the drug back into the gut lumen and cytochrome P450 metabolism. The presence of food and/or beverages can also interfere with absorption and bioavailability.
Bisphosphonates are a family of drugs used to prevent and treat bone fractures, osteoporosis, Paget's disease, metastatic bone cancer, and other bone diseases with high bone resorption. Bisphosphonates bind to bone hydroxyapatite and slow down bone-eroding cells known as osteoclasts. This effect allows the bone-building cells known as osteoblasts to work more effectively.
Some of the limitations with conventional bisphosphonates include irritation of the upper GIT, such as esophageal ulcers, and low bioavailability. As a result, conventional bisphosphonates require a specific dosing regimen so that the patient can absorb some of the drug properly and avoid side effects. Because many foods, beverages, medications and calcium interfere with absorption, conventional bisphosphonates must be administered on an empty stomach and, depending on the particular bisphosphonate, must wait from 30 minutes to two hours before consuming any food, beverages (other than water), medications or calcium supplements. As esophageal ulcers are a known side effect, dosing regimens for conventional bisphosphonates specify that patients consume an entire glass of water with the dosage form and avoid assuming a horizontal orientation, such as by lying down, within 30 to 60 minutes after administration.
The specific characteristics of alendronate served to exemplify the members of the class of bisphosphonates and the issues associated with them. Alendronate is a white, crystalline, odourless, non-hygroscopic bisphosphonate prepared by chemical synthesis. Alendronate monosodium trihydrate has a molecular weight of 325.1. Alendronate is approved in the U.S. for the prevention and treatment of osteoporosis in men and postmenopausal women, and for the treatment of Paget's disease of bone and glucocorticoid induced osteoporosis in both sexes. Like other bisphosphonates, alendronate binds to bone hydroxyapatite and specifically inhibits the activity of osteoclasts. Alendronate reduces bone turnover in human and animal models and decreases activation frequency, reducing bone resorption in both cortical and trabecular bone and ultimately increasing bone density and strength.
The oral bioavailability of alendronate is very low and independent of the dose (5-80 mg), averaging 0.76% in women and 0.59% in men. Presystemic metabolism does not occur. Following oral administration of conventional forms of alendronate, 40% of the dose absorbed is excreted in the urine within 8 hours and a further 5% is excreted over the next 64 hours. Sixty to seventy per cent of the absorption occurs within 1 hour of dosing. Bioavailability is markedly reduced by coincident consumption of food (85%-90%) and even consumption of coffee or orange juice will impair absorption by as much as 60% or more. Coincident medication will also reduce absorption, as any calcium-containing compounds and multivalent cations will bind to the bisphosphonate. Elevation of gastric pH above 6 is associated with a twofold increase in alendronate absorption. Alendronate is not metabolized and is excreted unchanged with renal clearance comparable to the glomerular filtration rate.
Bisphosphonate compositions and oral dosage forms with improved systemic bioavailability which are not subject to the dosing restrictions of conventional bisphosphonates would represent a considerable benefit for patients. As a result, new strategies for delivering drugs across the GIT cell layers are needed, particularly for bisphosphonates.
Numerous potential absorption enhancers have been identified. For instance, medium chain glycerides have demonstrated the ability to enhance the absorption of hydrophilic drugs across the intestinal mucosa (Pharm. Res. (1994), 11, 1148-54). However, the importance of chain length and/or composition is unclear and therefore their mechanism of action remains largely unknown. Sodium caprate has been reported to enhance intestinal and colonic drug absorption by the paracellular route (Pharm. Res. (1993) 10, 857-864; Pharm. Res. (1988), 5, 341-346). U.S. Pat. No. 4,656,161 (BASF AG) discloses a process for increasing the enteral absorbability of heparin and heparinoids by adding non-ionic surfactants such as those that can be prepared by reacting ethylene oxide with a fatty acid, a fatty alcohol, an alkylphenol or a sorbitan or glycerol fatty acid ester.
U.S. Pat. No. 5,229,130 (Cygnus Therapeutics Systems) discloses a composition which increases the permeability of skin to a transdermally administered pharmacologically active agent formulated with one or more vegetable oils as skin permeation enhancers. Dermal penetration is also known to be enhanced by a range of sodium carboxylates [Int. J. of Pharmaceutics (1994), 108, 141-148]. Additionally, the use of essential oils to enhance bioavailability is known (U.S. Pat. No. 5,665,386 AvMax Inc. and others). It is taught that the essential oils act to reduce either, or both, cytochrome P450 metabolism and P-glycoprotein regulated transport of the drug out of the blood stream back into the gut.
Often, however, the enhancement of drug absorption correlates with damage to the intestinal wall. Consequently, limitations to the widespread use of GIT enhancers are frequently determined by their potential toxicities and side effects. Additionally and especially with respect to peptide, protein or macromolecular drugs, the “interaction” of the GIT enhancer with one of the transport pathways should be transient or reversible, such as a transient interaction with or opening of tight junctions so as to enhance transport via the paracellular route.
As mentioned above, numerous potential enhancers are known. However, this has not led to a corresponding number of products incorporating enhancers. One such product currently approved for use in Sweden and Japan is the Doktacillin™. suppository [Lindmark et al. Pharmaceutical Research (1997), 14, 930-935]. The suppository comprises ampicillin and the medium chain fatty acid, sodium caprate (C10).
Provision of a solid oral dosage form which would facilitate the administration of a drug together with an enhancer is desirable. The advantages of solid oral dosage forms over other dosage forms include ease of manufacture, the ability to formulate different controlled release and extended release formulations and ease of administration. Administration of drugs in solution form does not readily facilitate control of the profile of drug concentration in the bloodstream. Solid oral dosage forms, on the other hand, are versatile and may be modified, for example, to maximize the extent and duration of drug release and to release a drug according to a therapeutically desirable release profile. There may also be advantages relating to convenience of administration increasing patient compliance and to cost of manufacture associated with solid oral dosage forms.